Qualcomm was Apple's exclusive supplier of modems for LTE and Wi-Fi connectivity, which explains why the iPhone 6s and previous generations supported both GSM and CDMA networks regardless of carrier. But with Intel entering the mix, choosing which iPhone to purchase is now more meaningful.
The lack of CDMA support in iPhones with Intel modems may be short lived, fortunately, as Intel acquired CDMA assets
from VIA Telecom in 2015. It is believed that this acquisition could pave the way for Intel to release its first LTE modem with both GSM and CDMA support as early as 2017 or 2018.
Ultimately, the most obvious choice should be to purchase a Verizon or Sprint model, even if you are an AT&T or T-Mobile customer, especially when considering resale value. However, some customers may have difficulty purchasing from another carrier, depending on their current status with their existing carrier.
Apple should also release an unlocked SIM-free model in the U.S., likely based on the Verizon/Sprint model, in a few weeks based on past launches.